Rechargeable prismatic batteries are used in a variety of industrial and commercial applications such as fork lifts, golf carts, uninterruptable power supplies, and electric vehicles.
Rechargeable lead-acid batteries are presently the most widely used type of battery. Lead-acid batteries are a useful power source for starter motors for internal combustion engines. However, their low energy density, about 30 Wh/kg, and their inability to reject heat adequately, makes them an impractical power source for an electric vehicle. An electric vehicle using lead acid batteries has a short range before requiring recharge, require about 6 to 12 hours to recharge and contain toxic materials. In addition, electric vehicles using lead-acid batteries have sluggish acceleration, poor tolerance to deep discharge, and a battery lifetime of only about 20,000 miles.
Nickel metal hydride batteries ("Ni--MH batteries") are far superior to lead acid batteries, and NI--MH batteries are the most promising type of battery available for electric vehicles. For example, Ni--MH batteries, such as those described in copending U.S. patent application Ser. No. 07/934,976 to Ovshinsky and Fetcenko, the disclosure of which is incorporated herein by reference, have a much better energy density than lead-acid batteries, can power an electric vehicle over 250 miles before requiring recharge, can be recharged in 15 minutes, and contain no toxic materials. Electric vehicles using Ni--MH batteries will have exceptional acceleration, and a battery lifetime of more than about 100,000 miles.
Extensive research has been conducted in the past into improving the electrochemical aspects of the power and charge capacity of Ni--MH batteries, which is discussed in detail in U.S. Pat. Nos. 5,096,667 and 5,104,617 and U.S. Pat. Nos. 5,238,756 and 5,277,999. The contents of all these references are specifically incorporated by reference.
Initially Ovshinsky and his team focused on metal hydride alloys that form the negative electrode. As a result of their efforts, they were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries capable of high density energy storage, efficient reversibility, high electrical efficiency, efficient bulk hydrogen storage without structural changes or poisoning, long cycle life, and repeated deep discharge. The improved characteristics of these "Ovonic" alloys, as they are now called, results from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix. Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage materials were formulated, based on the disordered materials described above. These are the Ti--V--Zr--Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which is incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti--V--Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti--V--Zr--Ni alloys are also used for rechargeable hydrogen storage negative electrodes. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
In contrast to the Ovonic alloys described above, the older alloys were generally considered "ordered" materials that had different chemistry, microstructure, and electrochemical characteristics. The performance of the early ordered materials was poor, but in the early 1980's, as the degree of modification increased (that is as the number and amount of elemental modifiers increased), their performance began to improve significantly. This is due as much to the disorder contributed by the modifiers as it is to their electrical and chemical properties. This evolution of alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688 (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference).
Simply stated, in all metal-hydride alloys, as the degree of modification increases, the role of the initially ordered base alloy is of minor importance compared to the properties and disorder attributable to the particular modifiers. In addition, analysis of the present multiple component alloys available on the market and produced by a variety of manufactures indicates that these alloys are modified following the guidelines established for Ovonic alloy systems. Thus, as stated above, all highly modified alloys are disordered materials characterized by multiple components and multiple phases, i.e. Ovonic alloys.
Clearly, the introduction of Ovonic alloying techniques has made significant improvements in the active electrochemical aspects of Ni--MH batteries. However, it should be noted that until recently the mechanical and thermal aspects of the performance of Ni--MH batteries have been neglected.
For example, in electric vehicles, the weight of the batteries is a significant factor because battery weight is the largest component of the weight of the vehicle. For this reason, reducing the weight of individual batteries is a significant consideration in designing batteries for electric powered vehicles. In addition to reducing the weight of the batteries, the weight of battery modules must be reduced, while still affording the necessary mechanical requirements of a module (i.e. ease of transport, ruggedness, etc.). Also, when these battery modules are incorporated into battery pack systems (such as for use in electric vehicles) the battery pack components must be as light weight as possible.
It should be particularly noted that electric vehicle applications introduce a critical requirement for thermal management. This is because individual cells are bundled together in close proximity and many cells are electrically and thermally connected together. Therefore, since there is an inherent tendency to generate significant heat during charge and discharge, a workable battery design for electric vehicles is judged by whether or not the generated heat is sufficiently controlled.
Sources of heat are primarily threefold. First, ambient heat due to the operation of the vehicle in hot climates. Second, resistive or I.sup.2 R heating on charge and discharge, where I represents the current flowing into or out of the battery and R is the resistance of the battery. Third, a tremendous amount of heat is generated during overcharge due to gas recombination.
While the above parameters are generally common to all electrical battery systems, they are particularly important to nickel-metal hydride battery systems. This is because Ni--MH has such a high specific energy and the charge and discharge currents are also high. For example, to charge a lead-acid battery in one hour, a current of 35 Amps may be used while recharge of a Ni--MH battery may utilize 100 Amps for the same one-hour recharge. Second, because Ni--MH has an exceptional energy density (i.e. the energy is stored very compactly) heat dissipation is more difficult than lead-acid batteries. This is because the surface-area to volume ratio is much smaller than lead-acid, which means that while the heat being generated is 2.5-times greater for Ni--MH batteries than for lead acid, the heat dissipation surface is reduced.
The following illustrative example is useful in understanding the thermal management problems faced when designing Ni--MH battery packs for electric vehicles. In U.S. Pat. No. 5,378,555 to General Motors (herein incorporated by reference), an electric vehicle battery pack using lead acid batteries is described. The battery pack system, utilizing lead-acid batteries, has a capacity of about 13 kWh, weighs about 800 pounds, and has a vehicle range of about 90 miles. By replacing the lead-acid battery pack by an Ovonic battery pack of the same size, the capacity is increased to 35 kWh and vehicle range is extended to about 250 miles. One implication of this comparison is that in a 15 minute recharge, the power supplied to the Ni--MH battery pack is 2.7 times greater than that supplied to the lead-acid battery pack, with its commensurate added heat. However, the situation is somewhat different during discharge. To power a vehicle on the highway at constant speed, the current draw upon the battery is the same whether it is a Ni--MH battery or a lead-acid battery (or any other power source for that matter). Essentially the electric motor which drives the vehicle does not know or care where it gets the energy or what type of battery supplies the power. The difference between the heating of the Ni--MH battery and the lead-acid battery upon discharge is the length of discharge. That is, since the Ni--MH battery will drive the vehicle 2.7 times farther than the lead-acid, it has a much longer time before it has a chance to "cool-off".
Further, while the heat generated during charging and discharging Ni--MH batteries is normally not a problem in small consumer batteries or even in larger batteries when they are used singly for a limited period of time, large batteries that serve as a continual power source, particularly when more than one is used in series or in parallel, such as in a satellite or an electric vehicle, do generate sufficient heat on charging and discharging to affect the ultimate performance of the battery modules or battery pack systems.
Thus, there exists a need in the art for battery, battery module, and battery pack system designs which reduces the overall weight thereof and incorporates the necessary thermal management needed for successful operation in electric vehicles, without reducing its energy storage capacity or power output, increases the batteries' reliability, and decreases the cost.